The use of patterns that are volatilized by the heat of molten metal poured into the cavity containing the pattern (such as polystyrene) has been known for some time. This process depends upon the use of dry, unbonded sand particles to form the mold walls. The minute crevices between the sand particles of the mold walls act as passageways for the hot gaseous or liquified products of the evaporation of the pattern to migrate out of the casting cavity.
Unfortunately the sand mold, being an insulator, sets up a temperature gradient which causes the volatilized products to quickly cool and condense on the sand particles of which the mold is constituted. Such condensation contaminates the sand for subsequent reuse in the casting process and must be subjected to costly independent reclamation procedures, which may or may not return the sand to its original condition. The contamination is particularly prevalent when casting metals having a liquidus below 2000.degree. F. because the decomposition products of the pattern more readily contain liquids which cannot be carried out from the sand by a gas exhaust.
The prior art has used gas flows, such as air, to fluidize the sand of the mold, principally for the purpose of allowing for the insertion of the pattern or the removal of the completed casting from the mold (see U.S. Pat. Nos. 3,581,802 and 3,557,867). The prior art has also used a heated positive gas flow, such as air, prior to the pouring of molten metal into a ceramic shell mold, the latter containing a vaporizable pattern. This latter technique softened the pattern only by using heated air flow at a stage before pouring of molten metal; the bulk of the pattern was subsequently removed by insertion of the pattern into a firing furnace, volatilizing the pattern, and allowing the volatilized gases to pass through the shell mold and out into the furnace atmosphere. However, since this procedure was conducted prior to the pouring of the molten metal into the cavity, the positive pressure of the air flow forced vapors that were to form, from the heat conducted thereinto, to pass outwardly through the neck of the ceramic shell mold and not pass through the mold itself into the surrounding sand body (see U.S. Pat. No. 3,222,738). The gases could not pass through the mold because of (a) the positive pressure of the heated air flow surrounding such mold, (b) the vaporized gases were limited in amount, and (c) the gases passed outwardly through the neck of the shell mold which was maintained above the top level surface of the surrounding sand medium. The problem of contaminated sand remains a problem since this technique fails to prevent condensation on sand during a metal pouring operation and because shell molds were not intended for reuse.